A conventional change of phase refrigeration system comprises a compressor for compressing relatively low pressure cool refrigerant gas into a relatively high pressure hot refrigerant gas, a condenser for cooling the hot refrigerant gas to a temperature below that at which it becomes a liquid while still remaining at the high pressure imposed at the output of the compressor, an expansion valve or metering device to control the flow of liquid refrigerant from the output of the condenser to an evaporator, the evaporator operating to refrigerate air circulating in the system by absorbing heat from air passing over the evaporator coils and thereby gasifying the liquid condensate contained therein. The evaporator gases are exhausted to a suction line which returns the gases to the compressor for recirculation through the system.
It is customary to operate systems of the type described above at conditions approximating summertime, whereby the discharge pressure of the compressor and, consequently, the condensing temperature of the condenser is relatively high. Thus, the compressor is run at a substantially high compression ratio.
However, the efficiency of the refrigeration system can vary substantially and it would be desirable to modify the components thereof to improve the efficiency of the system. For example, it is typical of compressors to increase in volumetric efficiency as the compression ratio of the discharge side of the compressor to the suction side of the compressor is reduced. With a compression ratio of 2-to-1 (discharge to suction), the efficiency of a refrigerating compressor is greatly increased (in the order of 90% or more). Further modifications to the refrigeration system would include a compressor which is cooled by the circulation of a relatively low temperature cooling medium, e.g., ambient air, therethrough to reduce the temperatures of the high pressure hot gases from the compressor to a temperature below that at which the gas becomes a liquid while remaining generally at the pressure imposed at the output of the compressor.
However, one factor militating against the use of compressors in a range producing lower discharge pressures and lower compression ratios, is the metering device commonly associated with a change of phase refrigeration system. Typically the metering device comprises a thermostatic expansion valve whose capacity responds to the difference between the pressure of the incoming condensate and the pressure of the outgoing gases from the evaporator. Typically these metering devices are adapted to function at relatively high condensing temperatures (conditions approximating summertime) because it is felt that there is a significant reduction in the ability of a metering device to pass sufficient refrigerant at lower condensing temperatures.